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| Titel |
How did the Dark Crater Surfaces in the Bright Areas of the Saturn Moon Iapetus Evolve? |
| VerfasserIn |
G. G. Galuba, T. Denk, G. Neukum |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028282
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| Zusammenfassung |
| The global brightness dichotomy of Iapetus has raised interesting questions since its
discovery by Giovanni D. Cassini in the year 1671: What is the bright material, what the
dark one? How did the dark areas evolve? Those questions are satisfyingly solved only for the
dark area covering two-fifth of the surface on the leading side, called Cassini Regio. It
has been suggested by Spencer (J. R. Spencer et al. 37th DPS Meeting 2005,
J. R. Spencer et al. 39th DPS Meeting 2007) that dark organic material is embedded in
ice on the whole surface of Iapetus, only some areas are suited for an enrichment
of dark material on the surface. A runaway process is started by a slight increase
in local heating. This enables increased sublimation of ice. A higher amount of
the dark organic material remains behind. This enrichment leads to a decreased
albedo and therefore increased absorption. The local temperature rises even more.
The Surface of Iapetus has areas where the runaway process did not occur – the
bright areas - and areas where the runaway process was triggered - those that are
darkened.
Within the large Cassini Regio the process in all probability was triggered by exogenous
material (T. Denk and J. R. Spencer 40th DPS Meeting 2008). A remaining question is how
the troughs und crater bottoms in the bright areas on the trailing side darkened. Here exogenic
material can be excluded and an intrinsic mechanism has to be found as a trigger for the
runaway process.
We suggest that the scattering of sunlight on the crater surface led to a local increase in
intensity on the inside of the crater comparable to the caustic of a concave mirror. This
increase triggered the aforementioned runaway process in case of the crater surfaces. As
Iapetus is a very slow rotator (sideric rotation period: 79.3Â days) the incidence angle for the
craters changes very slowly with time and the surface enters a local equilibrium between
absorption and reemission and heat conduction. Models of the irradiation, reflectance and
absorption using geometrical optics and different scattering models were used for the
calculations. The crater surfaces were modeled by facets on a grid. We will present models
of the increased local intensity that explain darkening of the surfaces of craters. |
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